Diesel engine manufacturers are continuously challenged to meet lower emission standards set forth by the U.S. Environmental Protection Agency (EPA), as well as other such agencies worldwide. These standards for both diesel and gasoline engines mandate limits for unburned hydrocarbons, carbon monoxide and nitrogen oxides (NOx). Current U.S. regulations for diesel engine emissions allow NOx emissions of only 4.0 g/bhp-hr. This will be reduced for the 2004 Model Year to a standard of 2.5 g/bhp-hr combined NOx and non-methane hydrocarbons emissions.
The undesirability of NOx compounds and their ability to further react to produce additional undesirable materials make them an undesirable by-product from the burning of hydrocarbons. These NOx compounds and their derivative reaction products comprise what is commonly referred to as “smog.”
Many methods have been used or suggested to reduce or eliminate NOx. A number of these rely upon reaction of NOx in the effluent exhaust gas in a system containing a reducing agent. Reducing agents such as ammonia, urea, and cyanuric acid have been used to selectively reduce NOx (NO+NO2) in the exhaust gas streams.
The NOx reduction steps in a effluent exhaust gas system can take place at low temperature over a catalyst, referred to as selective catalytic reduction (SCR), or at high temperature without the aid of a catalyst (selective non-catalytic reduction, or SNCR).
A recent example of SCR can be found in U.S. Pat. No. 6,203,770 B1. This patent describes the pyrolysis of urea (CO(NH2)2) in a chamber generating ammonia (NH3) and isocyanic acid (HNCO). These components are then mixed with NOx containing exhaust gases from a diesel engine and contacted with an SCR catalyst resulting in the reduction of NOx compounds.
Some in-cylinder technologies for reducing NOx have also been developed, such as exhaust gas recirculation. One way of implementing this method involves recirculating a portion of the exhaust gases back through the engine using pressure pulses created by the exhaust valves. The exhaust gases go through a cooler before being introduced back into the engine through the inlet. These gases dilute the air/fuel charge thereby lowering peak combustion temperatures and lowering NOx emissions.
All of these technologies require the design and implementation of additional systems for the exhaust gas, which increases costs and complexity, while often reducing engine efficiency.
Another limitation, that involves the SNCR method, is the requirement of a very high temperature, much higher than typical diesel exhaust gas temperatures.
It would be very advantageous to find a method of reducing problematic NOx emissions from a diesel engine that would not require expensive modifications to the exhaust system of diesel engines.